Originally published in Compte Rendus, Lab. Carlsberg Serie Chimique 25, 213 



(1940). 



93. EXCHANGE OF NITROGEN ATOMS IN THE LEAVES 



OF THE SUNFLOWER 



G. Hevesy, K. Linderstr0m-Lang, A. S. Keston and 



Carsten Olsen 



From the Institute for Theoretical Physics and the Carlsberg Laboratories of 



Copenhagen 



Making use of radioactive phosphorus as an indicator we found, in an 

 earlier investigation, that the greater part of the phosphorus atoms 

 present in the leaves and the roots of the plants investigated is not 

 permanently on one place but migrates from leaf to leaf (Hevesy, 

 Linderstr0m-Lang and Olsen, 1936, 1937). We investigated later, on 

 similar lines, the behaviour of nitrogen. The investigation of the be- 

 haviour of nitrogen seemed to be of great interest, since the greatest part 

 of nitrogen is present in the plants in the form of protein compounds, 

 and the replacement of such nitrogen atoms by others involves the 

 bracking of nitrogen bonds and a restitution of protein molecules by 

 enzymatic action. 



Nitrogen has no known radioactive isotope of suitable life; however, 

 the heavy nitrogen isotope ^^N can be used as an indicator. Through 

 the great kindness of Professor Urey, we came into possession of large 

 amounts of ammonium chloride, which contained i^N in a very appre- 

 ciable excess over that present in ordinary ammonium chloride. While 

 the nitrogen of the air contains 0.37 per cent. ^^N, Professor Urey's 

 preparation contained 2.5 per cent. 



Sunflowers were grown in an ordinary culture solution containing 

 normal ammonium sulphate until a set of leaves, the lower ones, appeared. 

 We then placed the plant in a second culture solution in which the nor- 

 mal ammonium content was replaced by "heavy" ammonium. After 

 the lapse of three weeks, we transformed the nitrogen of the plants into 

 ammonia by Kjeldahl's method. The "lower" leaves, which have 

 grown only to a minor extent during the second stage of the experiment, 

 and the "upper" leaves, which were to a very large extent formed 

 during the second stage of the experiment, were separately treated. 

 The upper leaves having been developed in a culture solution containing 

 ^^N were bound to contain a corresponding amount of the heavy nitrogen 

 isotope. In the "lower" leaves, ^^N could, however, only enter in excess 



